Air-purifying respirators are used in a variety of applications to protect the user from inhaling potentially life-threatening concentrations of compounds that can have adverse health effects with prolonged exposure. Typically, such air-purifying respirators include at least one activated carbon filtration element to remove toxic gases such as ammonia from the user's airstream. Though activated carbon can be used with some success, it has limitations, particularly in dry and humid conditions. Traditional extended surface area substrates typically must be enhanced by reducing the particle size distribution of materials. This can result in increased breathing resistance or the potential for reducing the particulate size of the adsorptive media such that it could become respirable and no longer retained in the filtration device.
Typically, ammonia adsorbents are high surface area materials such as activated carbons or zeolites. Due to the low boiling point of ammonia, these materials are not suitable to trap ammonia vapors irreversibly. Zeolites, with their small micropore sizes, are better at trapping ammonia; however, since zeolites and high activity activated carbon alone do not destroy the compound, it is possible for the adsorbent to release the ammonia. To overcome this problem, most adsorbents are impregnated with a chemical that can react and neutralize the ammonia vapors. Most commonly, these materials are acids (citric acid, phosphoric acid, or sulfuric acid) or transition metal salts (Cu, Zn).
U.S. Pat. No. 7,309,513 B2, December 2007, Brey et al. describes a broad-spectrum filter media that use Cu and Zn salts as ammonia-sensitive impregnants. The adsorbent also is impregnated with ammonium sulfate that can decompose to form an acidic material. GC-IPA from General Carbon Corporation (Paterson, N.J., USA) employs phosphoric acid (up to 45%) on coal-based carbon to neutralize ammonia. PICA USA, Inc. (Columbus, Ohio, USA) uses a coconut shell activated carbon impregnated with zinc chloride to adsorb ammonia (Picatox AM80).
High surface area materials (carbon, alumina, zeolites) are commonly used for air filtration because they can remove many contaminants. The ability of these materials to remove certain chemicals greatly depends on the chemical composition of the adsorbent, surface area, and pore size. The removal also depends on the chemical and physical properties of the contaminant. These properties determine whether a contaminant is physisorbed or chemisorbed. Physisorbed contaminants are usually only weakly bound and can be released with changes in temperature. Chemisorbed materials undergo a chemical reaction on the adsorbent surface and are thus converted to a different material. Activated carbons, while containing very high surface areas, are insufficient to adsorb low boiling point compounds such as ammonia, thus some type of impregnant is required to react and trap the contaminant. Impregnation of any support involves a balance of loading a sufficient amount of the reactive impregnant without destroying the high surface area of the support. Some types of metal oxides have shown the capability to adsorb ammonia, but high surface area materials are necessary to adsorb any significant amount.
There remains a need for an improved filtration medium for removing ammonia from air streams.